The present invention relates generally to a receiver for digital communications over any media and, more specifically, to a method and system for removing jitter from demodulated data streams subject to variable delays and for correcting timestamps to accurately reflect the temporal location of selected packets relative to other packets.
There have long been needs in the art to transmit real-time media signals, such as, voice, audio and/or video, from one location to another using digital communications systems, such as, hybrid fiber coax (HFC), satellite and terrestrial broadcast systems. At the receiver locations, these media signals require synchronization of the time reproduction in order to allow such signals to be rendered suitable for listening and/or viewing.
Under conventional prior art approaches, the time information, such as a sequence number, timestamp or clock reference, is inserted into the packetized media data. For example, MPEG (moving picture experts group) receiver systems synchronize their internal clocks to received timestamp values, known as program clock reference (PCR) values, by sampling the PCR values, calculating the difference between the received PCR values and an internal clock value, and then synchronizing the internal clock according to the calculated difference. However, variable delays may be introduced. These delay variations include, for example, demodulating/decoding time in demodulators, channelized delays in multi-channel systems, and packetization jitter. Variations in arrival times to a PCR sampler produce PCR jitter and degrade the quality of service (QoS).
Under another approach used to reduce jitters, both transmitters and receivers use a common clock source, such as, global positioning systems. By synchronizing to the common clock, it is possible for a receiver to create a substantially accurate timestamp with a transmitter. However, both the transmitter and the receiver have to be equipped with global positioning system receivers. A global positioning system receiver includes an antenna, associated circuitry and/or software which may increase the system complexity and cost.
Hence, it would be desirable to have a system that is capable of providing jitter-free transmissions in a more efficient manner.